Photonic Modulator and Switch

ABSTRACT

The invention taught herein provides a method, device and system for modulating or switching electromagnetic radiation by controlling a state of the radiation, such as a polarization state. Radiation is directed at a reflective or transmissive structure, such that the radiation is incident on the structure. The structure includes a property that can be dynamically switched between two configurations, one of which is asymmetric and is configured to modify the polarization characteristic of the radiation. The dynamically configurable structure can be combined with polarization components to achieve modulation. Embodiments suitable for mode-locking a laser and for cavity dumping a mode-locked laser are also disclosed.

RELATED APPLICATIONS

This U.S. patent application, docket number JH170612DIV is a divisionalof U.S. patent application Ser. No. 14/014,257 (docket numberJH130805US) filed on Aug. 29, 2013 and claims priority from provisionalpatent application 61/694,765 with docket number JH091103PR which wasfiled on Aug. 30, 2012.

FIELD OF THE INVENTION

This US patent application relates to switching electromagneticradiation between at least two states. More generally it relates tomodulating electromagnetic radiation. Modulating or switchingelectromagnetic radiation and in particular optical signals hasapplications in multiple fields including, but not limited to, opticalcommunications; optical data storage; optical imaging and analysis;generation of a train of optical pulses; and extracting a single opticalpulse.

BACKGROUND OF THE INVENTION

Modulating electromagnetic radiation, and in particular switchingoptical signals between two or more intensities or phases is used toencode information on an optical signal for purposes such ascommunication or storage of information. The amount of information thatcan be encoded is related to the rate or frequency at which modulationcan be performed.

At relatively low modulation rates, optical modulation can be performedby, for example, modulating the current to a laser diode. Modulating thecurrent to turn the laser diode on or off is often referred to as on-offkeying (OOK). Such on-off current modulation generates an intensitymodulated optical signal.

To achieve intensity modulation at very high modulation rates a commontechnique is to separate the optical signal into two components, phasemodulate at least one component such that a 180 degree relative phaseshift can be introduced between the two components which are thenrecombined. Modulating the phase in this manner results in an intensitymodulated optical signal, as in a conventional Mach Zehnder modulator.

Another conventional technique for modulating or switching an opticalsignal is to use a Pockels cell, which is a voltage controlledwave-plate based on the electro-optic effect. A Pockels cell istypically combined with a polarizer and can switch the plane ofpolarization of an optical signal between zero optical rotation and 90°rotation at high speeds. Such a rotation enables redirecting the opticalsignal by means of a polarized beam splitter.

Operation of a Mach Zehnder modulator or a Pockels cell modulatordepends on weak bulk material effects of the phase modulating materialof the Mach Zehnder modulator and the voltage controlled wave-platematerial of the Pockels cell. The requirement of modifying bulkproperties of a material to achieve modulation has speed limitingconsequences and places a significant burden on the manufacturing andoperation of these devices, requiring a Mach Zehnder modulator to bephysically long and requiring high voltage for operation of a Pockelscell modulator. These burdens have negative physical size and costconsequences.

There is therefore an unmet need for a method, apparatus and system forhigh speed a optical modulator or switch that is not limited by speedlimitations of bulk material based modulators and is not limited by thephysical size and cost issues of bulk material based modulators.

SUMMARY OF THE INVENTION

The invention taught herein provides a method, device and system formodulating or switching electromagnetic radiation by controlling apolarization characteristic of the radiation, by directing the radiationat a reflective or transmissive structure, such that the radiation isincident on the structure. The structure includes a property that can bedynamically switched between two configurations, one of which isasymmetric and is designed to modify the polarization characteristic ofthe radiation. The dynamically configurable structure can be combinedwith polarization components to achieve modulation. An embodimentsuitable for mode-locking a laser is also disclosed. An embodimentsuitable for cavity dumping a mode-locked laser is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an illustration of a preferred embodiment in which adynamically configurable reflective element in combination with apolarized beam splitter comprises an optical modulator according to theinvention. FIG. 1B is an illustration of an example of a modulatingsignal.

FIG. 2A, FIG. 2B, FIG. 2C and FIG. 2D illustrate in more detailed thedynamically configurable reflective element.

FIG. 3A is a detailed illustration of the preferred embodiment of thedynamically configurable reflective element. FIG. 3B is a side view ofthe preferred embodiment of the dynamically configurable reflectiveelement.

FIG. 4 illustrates electronic circuit aspects of the preferredembodiment of the dynamically configurable reflective element.

FIG. 5A is an illustration of an alternative embodiment of thedynamically configurable reflective element. FIG. 5B is a side view ofan alternative embodiment of the dynamically configurable reflectiveelement.

FIG. 6A, FIG. 6B and FIG. 6C illustrate another alternative embodimentof the dynamically configurable reflective element.

FIG. 7A and FIG. 7B illustrate yet another embodiment of the inventivesystem suitable for applications such as: (a) mode-locking a laser, (b)cavity dumping a laser.

FIG. 8A is an illustration of a modulating signal associated with amode-locked laser. FIG. 8B is an illustration of the resultingmode-locked optical pulse train. FIG. 8C is an illustration of amodified modulating signal. FIG. 8D is an illustration of the resultingmode-locked optical pulse output from the laser cavity due to modulatingsignal in FIG. 8C.

FIG. 9A, FIG. 9B and FIG. 9C illustrate an embodiment of the inventivesystem where the dynamically configurable element consists of an induceddiffraction grating that is imposed by an optically generatedinterference pattern.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of this invention is illustrated in and describedwith reference to FIG. 1A. The preferred embodiment includes an opticalsource 101, that generates collimated linearly polarizedelectro-magnetic radiation 103 that is applied to an optionalpolarization optic 105. The optional polarization optic 105 is apolarizer that enhances the degree of linear polarization of theradiation 103. The optional polarization optic 105 may also include awave-plate to rotate the plane of polarization.

The linearly polarized radiation 123, is applied to a polarized beamsplitter 107 that directs substantially all the radiation 109 of onelinear polarization with a first polarization vector through thepolarized beam splitter 107 towards a reflective element 111 alsoreferred to herein as a reflective structure. Any residual radiation ata polarization angle orthogonal the first linear polarization vector isdirected out of the system along the line labeled 113 (as is any leakageat the first polarization vector angle) or to an optional monitoringdetector 115. The radiation 109 can be considered as a first ensemble ofphotons with a first polarization vector,

The radiation 109 is incident on the reflective element 111 at normalincidence and, in the preferred embodiment, a substantial amount of theincident radiation 109 is reflected back along the same line asindicated by the line of the radiation 109.

The reflective element 111 has a property that can be dynamicallyconfigured by an electronic signal 117. In a first configuration of thereflective element 111, the dynamically configurable property issymmetric in that it reflects incident radiation with a polarizationvector of any orientation with substantially the same high reflectivity.In this symmetric configuration the reflective element 111 hassubstantially no effect on the linear polarization vector of theradiation 109. In this first symmetric configuration, because there isno change in polarization upon reflection, the reflected radiation isdirected back through the polarized beam splitter 107 back towards theoptical source 101.

In this first configuration of the reflective element 111 there issubstantially no radiation redirected by the polarized beam splitter 107along the output direction indicated by 113 (because there is no changein polarization caused by the reflective element 111). In thisconfiguration, the actual amount of radiation redirected by thepolarized beam splitter 107 is determined by the extinction ratio of thepolarized beam splitter 107. Even a low cost consumer polarized beamsplitter can be very high extinction ratio.

In a second configuration of the reflective element 111, the dynamicallyconfigurable property is asymmetric in that the reflectivity of thereflective element 111 is different for incident radiation of differentpolarization vector orientation. In the preferred embodiment thereflective element 111 would have a first reflectivity if orientatedparallel to the polarization vector of the incident radiation and wouldhave a second different reflectivity if orientated perpendicular (ororthogonal) to the polarization vector of the incident radiation. In thepreferred embodiment the first reflectivity is a high reflectivity ofgreater than 97.5% and the second reflectivity is greater than 95% butless than the first reflectivity.

In the preferred embodiment the asymmetry of the second configuration ofthe reflective element 111 is a periodic perturbation that can bedynamically induced. The periodic perturbation can be considered as adiffractive grating, also referred to as a diffractive element, whichdiffracts a small amount of incident radiation with a polarizationvector perpendicular to the effective lines of the diffractive element,thereby reducing the reflectivity for incident radiation with thispolarization vector.

The dynamically induced diffractive element has substantially no effecton incident radiation with a polarization vector parallel to theeffective lines of the diffractive element, thereby enabling higherreflectivity for incident radiation with this polarization vector thanfor incident radiation with a polarization vector perpendicular to theeffective lines of the diffractive element.

In the preferred embodiment the diffractive element is azimuthallyaligned at an angle of 45 degrees with respect to the linearpolarization vector of the radiation incident on the reflective element111. This alignment of the diffractive element causes the incidentradiation to behave as if 50% of the photons have a polarization vectorperpendicular to the effective lines of the diffractive element 50% ofthe photons have a polarization vector parallel to the effective linesof the diffractive element.

Incident radiation can be considered as an ensemble of photons. Thepolarization vector of an incident ensemble of photons can be consideredas a quantum state of the incident radiation. When the incident ensembleof photons encounters the diffractive element azimuthally aligned at 45degrees with respect to the polarization vector of the incidentradiation, the diffractive element causes the quantum state of incidentphotons to switch to one of two quantum states both of which havepolarization vectors at an angle of 45 degrees to the polarizationvector of the incident radiation. The polarization vectors of the twosets of photons have orthogonal polarization vectors, i.e. that are atan angle of 90 degrees with respect to each other.

This switching to one of two quantum states results in the incidentensemble of photons with a first polarization vector, that can beconsidered as a first quantum state, being resolved into two reflectedensembles of photons, with different polarization vectors from eachother and with polarization vectors that are different from the firstpolarization vector of the incident radiation. The resulting twopolarization vectors of the reflected radiation can be also consideredas quantum states different from the quantum state of the incidentradiation.

The occurrence or non-occurrence of this quantum state switching from anincident first polarization vector of the incident radiation to at leastone second different polarization vector of the reflected radiation isdependent on the presence or absence of the diffractive element and issubstantially insensitive to the magnitude of the diffraction effect (ordiffraction efficiency) of the diffractive element.

This insensitivity to the magnitude of the diffraction effect is of bothpractical and of philosophical interest. In contrast, conventionaloptical switches, such as a Pockels cell, rely on a relatively weakelectro-optic effect and application of a high voltage to incrementallyrotate a polarization vector as radiation propagates through asignificant volume of such a cell. In the case of a Pockles cell themagnitude of the rotation is sensitive to the magnitude of theelectro-optic effect and typically requires propagating through asignificant length of material to accomplish a significant polarizationvector rotation.

In the preferred embodiment the magnitude of the diffraction efficiencydoes determine the decrease in intensity of the radiation associatedwith one reflected ensemble of photons. In the preferred embodiment thediffraction efficiency of the induced diffractive element is smallresulting in an intensity decrease of a reflected ensemble of photons ofnot greater than 2.5%. In other embodiments the induced diffractiveelement could have higher diffraction efficiency. Furthermore therelative phase of the two reflected ensembles can be controlled bydesign.

Referring again to FIG. 1 with the reflective element 111 in the secondconfiguration. The reflected radiation, with an angle of substantially45 degrees with respect to the polarization vector of the radiationincident on the reflective element 111, is reflected back to thepolarized beam splitter 107 and approximately 50% of this reflectedradiation is redirected by the polarized beam splitter 107 along thedirection indicated by 113 which is the output direction.

The remaining approximately 50% of the reflected radiation is directedby the polarized beam splitter 107 back towards the optical source 101.In situations where the optical source can be adversely affected byradiation being sent back to the optical source, the source can beisolated from such back directed radiation using conventional isolationtechniques.

In this second configuration of the reflective element 111, there is asubstantial amount of radiation redirected by the polarized beamsplitter 107 along the output direction indicated by 113. That is, asubstantial amount of radiation is switched in the direction indicatedby 113.

The electronic signal 117 switches the reflective element 111 betweenthe first and second configurations and thereby switches between (a)redirecting substantially no radiation by the polarized beam splitter107 along the output direction indicated by 113; and (b) redirectingapproximately 50% of the radiation reflected back to the polarized beamsplitter 107, i.e. the electronic signal modulates or switches theradiation output along the output direction indicated by 113.

An example of a OOK type modulating signal is illustrated in FIG. 1Bwhere the voltage at a first level indicated by 119 is at a firstvoltage, V1, (which in the preferred embodiment is ground voltage). Whenthis first voltage is applied to the dynamically configurable asymmetricproperty of the reflective element 111 (by way of the electronic signal117 which is also referred to as Vmod), it has no effect on the linearpolarization of the radiation 109. In this configuration the reflectedradiation is directed through the polarized beam splitter 107 backtowards the optical source 101.

In the situation where the voltage applied to the dynamicallyconfigurable property is at a second different level, V2, indicated by121, which puts the dynamically configurable property of the reflectiveelement 111 into a second configuration that is an asymmetricconfiguration. In this configuration the linear polarization vector ofthe reflected radiation is at an azimuthal angle of substantially 45degrees with respect to the polarization vector of the radiationincident on the reflective element 111 (i.e. the first ensemble ofphotons).

In this asymmetric configuration of V2, the resulting reflectedradiation is redirected by the polarized beam splitter 107 along thedirection indicated by 113 which is the output direction. Thus theradiation is intensity modulated in a manner determined by theelectronic modulating signal 117. The combination of the polarized beamsplitter 107 and the dynamically configurable reflective element 111comprises an optical modulator and is also referred to as a photonicswitch.

The mechanism by which the electronic signal 117 switches the reflectiveelement 111 of FIG. 1A between the first and second configurations isillustrated in and described with respect to FIG. 2. The dynamicallyconfigurable reflective element 111 (of FIG. 1A) is shown in more detailin FIG. 2A, FIG. 2B and FIG. 2C.

FIG. 2A depicts the incident radiation 201 passing through a substrate203 to the reflecting element 205. An expanded view (not to scale) ofFIG. 2A is shown in FIG. 2B where again the incident radiation 207passes through the substrate 209 and is substantially reflected by afirst layer 211 that is optically highly reflective and has specificelectrically conductive properties.

A second layer 213, that is an electrical insulating layer, electricallyisolates the first layer 211 from a third layer 215. The third layer 215is shown in more detail in FIG. 2C where a bottom view in the circle 217depicts a patterned electrically conductive element that consists of acombination of large pads, such as 219, and a set of separatedconductive lines, one of which is indicated by 221.

When an electrical current flows in the conductive lines, such as 221,it induces electrical behavior in the first layer 211 of FIG. 2B. Thedirection 223 of the patterned conductive lines, such as 221, is rotated(azimuthally) by approximately 45 degrees from the direction of thepolarization vector 225 of the incident radiation.

The induced electrical behavior in the first layer 211 is asymmetric andis also aligned with the direction 223. This induced asymmetric propertyin the first layer 211 generates an effective diffractive element inthis first layer 211. The induced diffractive element modifies thepolarization vector of the incident radiation such that the reflectedradiation has different polarization characteristics.

The polarization vector of the incident radiation is illustrated in FIG.2D, as 227. The vector 227 can be resolved into component 229 (parallelto the direction 223 of the conductive lines 221) and component 231perpendicular (or orthogonal) to direction 223.

The incident radiation is composed of a collection of photons, alsoreferred to as an ensemble of photons. The ensemble of photons has apolarization vector determined by the polarized beam splitter 107 ofFIG. 1A. When the ensemble of photons encounters the reflective element111 in the first configuration, i.e. the symmetric configuration, theensemble of photons is reflected with its polarization vectorsubstantially unchanged.

When the ensemble of photons encounters the reflective element 111 inthe second configuration, i.e. the asymmetric configuration, theensemble of photons is resolved into two ensembles of photons. The firstof the two ensembles of photons has a polarization vector 229 parallelto the direction 223. The second of the two ensembles of photons has apolarization vector 231 perpendicular (or orthogonal) to the direction223.

When the two reflected ensembles of photons with polarization vectors229 and 231 encounter the beam splitter 107 of FIG. 1 they are resolvedinto components one of which is parallel to the incident polarizationvector 227 and the other of which is perpendicular to the incidentpolarization vector 227.

The parallel components pass through the beam splitter 107 back towardsthe optical source while the components perpendicular to the incidentpolarization vector 227 are redirected by the beam splitter 107 alongthe direction 113 of FIG. 1. In this manner the radiation propagatingout of the system along direction 113 is intensity modulated by themodulating voltage applied to the patterned electrically conductiveelement.

The magnitudes of the two components 229 and 231 of FIG. 2D are depictedas being substantially the same value, however, in the preferredembodiment, the magnitude of one component will be diminished by themagnitude of the radiation that is diffracted by the induced asymmetricproperty of the diffractive element in the first reflective layer 211.

While in the preferred embodiment the first reflectivity is a highreflectivity of greater than 97.5% and the second reflectivity isgreater than 95% but less than the first reflectivity, in anotherembodiment the second reflectivity is less than 95% and in yet anotherembodiment the magnitude of the second reflectivity is negligible.

Whether the reflected radiation consists of one ensemble of photons withthe same polarization vector as that of the incident radiation orwhether the reflected radiation consists of one or two ensembles ofphotons with polarization vectors different from that of the incidentradiation depends on the presence or absence of the diffractive elementand is substantially independent of the magnitude of the diffractiveeffect.

This substantial insensitivity to the magnitude of the diffractiveeffect enables achieving significant depth of modulation with weakmodulating signals and thereby enables high speed operation and reducedsensitivity to noise on the modulating signal.

Depending on the specific application, the magnitude of the inducedasymmetric property required to cause the incident ensemble of photonsinto two reflected ensembles of photons may be minimal, resulting inminimal diffracted radiation. Alternatively the magnitude of the inducedasymmetric property can be large in order to significantly diminish themagnitude of one reflected ensemble. Furthermore the reflective elementcan be designed to achieve a desired relative phase of the two reflectedensembles of photons.

A more detailed illustration of the preferred embodiment of thedynamically configurable reflective element is depicted in FIG. 3A. Thefirst layer 211 of FIG. 2B is depicted in FIG. 3A as 301 and in thepreferred embodiment is comprised of a conductive material such as:gold; silver; copper; etc. or semi-conducting materials such as silicon.An electrically insulating second layer 303 separates the first layer301 from the third patterned layer 305 also comprised of a conductivematerial such as: gold; silver; copper; etc.

The patterned layer 305 includes interleaved conductive lines. Anexample of such a conductive line is 307. In the preferred embodimentone end of the conductive lines is connected by means of a through-holeor via to the first layer 301. Such through-holes are depicted by thecircular ends of the lines, an example of which is depicted as 309.

A side view of the layers in relation to the substrate is depicted inFIG. 3B where the substrate 311, the first conducting layer 313, theinsulating layer 315 and the patterned conducting layer 317 are allillustrated.

A schematic diagram illustrating electronic circuit aspects of thepreferred embodiment of the dynamically reconfigurable reflectiveelement is depicted in FIG. 4. The first conducting layer 401, theinsulating layer 403 and the patterned conducting layer 405 are againillustrated. The first conducting layer 401 is connected to ground asindicated by 407. The patterned conducting layer 405 is connected to themodulating voltage Vmod indicated by 409.

As described earlier, the modulating voltage Vmod, indicated by 117,switches between a voltage V1 which in the preferred embodiment isground and a different voltage V2. When the V2 voltage is applied to thepatterned layer a current flows in one direction in the set of lines, ofwhich 411 is one. The line 411 is connected to ground by way of thethrough-hole 413. Meanwhile a current will flow in the oppositedirection flows in the second set of lines, of which 415 is one.

These counter flowing currents induce a periodic electrical pattern inthe first conductive layer 401 which constitutes an asymmetric propertyof the reflective layer 401. The asymmetric property persists only aslong as the non-zero voltage is applied to the patterned layer 405.

Ideally the incident radiation should encounter substantially the sameasymmetric property at all regions of the cross-section of the beam andso should be confined to the region indicated by the dashed circle. Theincident radiation may be focused within the dashed circle in order tominimize the area of the asymmetric property in order to optimizemodulation speed or for other reasons.

It should be understood that the schematic diagram depicted in FIG. 4 isfor illustrative purposes and not intended to be to scale or to be anaccurate or complete depiction of the electronic aspects. Manyconsiderations, such as modulation speed will determine an appropriatephysical layout.

Embodiments other than the preferred embodiment are included in thisinvention. An alternative embodiment is illustrated in FIG. 5A. In manyrespects this embodiment is similar to that depicted in FIG. 3A, FIG. 3Band FIG. 4, however in this embodiment there are two addition layers andthe first layer 301 of FIG. 3A and 313 of FIG. 5B is not connected bythrough-holes to the patterned layer 305 of FIG. 3A and 315 of FIG. 5B.

The two additional layers consist of an insulating layer 501 and aconductive layer 503. These layers are shown in side view as layers 505and 507 respectively. In this embodiment the ends of the lines of thepatterned layer 305 are connected by through-holes to the additionalconductive layer 503. One such through-holes is indicated by 509.

In this embodiment the modulating voltage is again applied to thepatterned layer 305. The additional conductive layer 503 is connected toground. The first conducting layer 301 may optionally be also connectedto ground.

A second alternative embodiment is illustrated in FIG. 6 which in manyrespects is similar to the first alternative embodiment described above.In this second alternative embodiment the incident radiation 601propagates through a substrate 603 that has a reflective dielectricstack 607 deposited on its base. The dielectric stack 607 is alsoreferred to as a reflective structure. Additional layers 609 and 611 arealso deposited and consist of a patterned conductive layer 609 and aconductive layer 611 with one or more insulating layers separating thepatterned layer from a conductive layer.

The conductive layer is connected to the ends of the conductive lines ofthe patterned layer by means of through-holes in a similar manner as inthe first alternative embodiment. In this second alternative embodiment,layer 611 of the dielectric stack is a conductive but opticallytransparent layer in which the asymmetric property is induced.

Many variations of this second alternative embodiment are possible. Forexample, layer 611 could consist of non-conducting material in which anasymmetric charge distribution is the asymmetric property induced by theelectric currents in the patterned layer. More than one 611 layer couldbe embedded in the dielectric stack. In yet another alternativeembodiment, such layers could have their refractive index modified togenerate one or more asymmetric refractive index distribution inresponse to the electric currents in the patterned layer.

In the preferred embodiment the modulating signal shown in FIG. 1B is adigital waveform with an abrupt transition between the high and lowvoltage values. The transition between the high and low voltage valuescause the transition between the symmetric and asymmetric configurationsof the dynamically configurable element which in turn causes thereflected radiation to consist of a single ensemble of photons with onepolarization vector or to consist of two ensembles of photons with twodifferent polarization vectors.

As previously indicated, switching to the state consisting of twoensembles of photons with two different polarization vectors depends onthe existence of the asymmetric configuration and is relativelyinsensitive to the magnitude of the asymmetry and therefore insensitiveto the magnitude of the high voltage. This insensitivity to themagnitude of the asymmetry has significant practical value.

One practical value of being insensitive to the magnitude of theasymmetry is that the speed of switching from one state to the other isless sensitive to the rise and fall time of the electronic signal. Asecond practical value is that the depth of modulation of the outputmodulated optical radiation 113 of FIG. 1 is determined by theextinction ratio of the polarized beam splitter 107 of FIG. 1 (or theextinction ratio of an equivalent optical element in other embodiments).

The modulating digital waveform of the preferred embodiment is suitablefor applications such as optical communications or optical storage,however, other modulating waveforms are suitable for other applications.An application involving mode-locking a laser is illustrated in FIG. 7Awhich includes a simplified schematic diagram of an end pumped laser.The laser cavity is comprised of the two reflective elements 701 and703.

The reflective element 701 is one end of a lasing material 705. Thereflective element 701 is highly transmissive at the pump wavelength ofthe pump radiation 707 emitted by the pump source 709. The pump source709 may, for example, be a laser diode. The reflective element 701 isalso highly reflective at the lasing wavelength or wavelength range.

The reflective element 703 is a dynamically configurable reflectiveelement on a substrate 711 as described earlier. With no modulatingsignal being applied to the dynamically configurable reflective element701 the reflective element 701 is highly reflective at the lasingwavelength or wavelengths which confines the lasing electromagneticradiation within the laser cavity.

In this first configuration of the dynamically configurable property ofthe reflective element 701 the polarization vector of electromagneticradiation of the laser cavity remains substantially unchanged uponreflection at the reflective element 701, thus allowing theelectromagnetic radiation to substantially remain in the laser cavity.This lasing radiation constitutes a first lasing ensemble of photonswith a first polarization vector.

The reflective element 701 is configured to be azimuthally aligned withrespect to the polarized beam splitter 719, located in the laser cavity,which is also referred to as a first polarized optical element such thatwhen the dynamically configurable element of the reflective element 701is in the second configuration then the first lasing ensemble of photonsis resolved into two second ensembles of photons with polarizationvectors different from the first lasing ensemble of photons.

A polarized beam splitter 719 located in the laser cavity is azimuthallyaligned with the preferred polarization vector of the lasing radiation717. This polarized beam splitter 719 will direct any residual radiationat a perpendicular or orthogonal polarization vector out of the cavityalong directions 731 and 721. Any radiation directed along the direction721 may be detected by an optional detector 723.

Applying the modulating signal 725 (also referred to as Vmod) of FIG. 7Bto the dynamically configurable reflective element 703 periodicallyswitches the reflective element 703 between its two configurations. Onlyphotons or photon ensembles whose polarization vector encounters thereflective element 703 while it is in its configuration determined bythe first voltage, V1, propagate back through the polarized beamsplitter 719 substantially unaffected by the polarized beam splitter719.

Photons or photon ensembles whose polarization vector encounters thereflective element 703 while it is in its configuration determined bythe second voltage V2, or some fraction thereof, propagate back throughthe polarized beam splitter 719 with a polarization vector componentthat will be directed by the polarized beam splitter 719 out of thelaser cavity along the direction of 731.

An electronic modulating signal suitable for mode-locking the laser ofFIG. 7A is illustrated in FIG. 7B. The electronic modulating signal 725is a sine wave type signal that periodically varies from one voltage(V1, which is typically ground) indicated by 727 to a different secondvoltage (V2). The period of the signal 729 is selected to match theround trip time of radiation in the laser cavity of FIG. 7A.

The electronic signal with the second voltage (V2) is applied,synchronously with the round trip time of the laser cavity, to thereflective element with a dynamically configurable property, such thatthe electronic signal configures the reflective element 703 in thesecond configuration whereby at least a portion of the lasing ensembleof photons is resolved into two second ensembles of photons withpolarization vectors different from the first lasing ensemble ofphotons.

This will cause at least a portion of the two second ensembles ofphotons to be directed by the polarized beam splitter 719, also referredto as a polarization element, out of the laser cavity. Thus the firstlasing ensemble of photons is loss modulated and since the modulatingvoltage (V2) is applied synchronously with the round trip time of thelaser cavity, it is a periodic loss modulation that has the effect ofmode-locking the lasing electromagnetic radiation of the laser cavity ina manner similar to conventional acousto-optic or phase modulationmode-locking that will be familiar to one skilled in the art.

Any residual photons with the same polarization vector (possiblyamplified by the lasing material 705) are directed out of the lasercavity along the direction of 721 by the polarized beam splitter 719.

FIG. 8A again depicts electronic mode-locking drive signal 801 thatmode-locks the radiation in the laser cavity of FIG. 7A. The next to toptrace 803 in FIG. 8B depicts the resulting mode-locked optical pulsetrain. The trace 803 shows the intensity profile of a time sequence ofoptical pulses, one of which is 805. The pulse width of such pulses isdetermined (in part) by the bandwidth of the lasing material 705 of FIG.7A.

By modifying the electronic modulating signal 713 of FIG. 7A, that isapplied synchronously with the round trip time of the laser cavity, sothat it is an extended electronic signal that remains active for atleast the duration of a mode-locked pulse in the manner depicted in FIG.8C where the electronic modulating signal remains at the high voltage V2for the time period between the dashed lines indicated by the doublearrow 809.

Applying the high voltage V2 for the time period 809 causes thepolarization vector of the pulse 807 of FIG. 8B to be at least in partswitched or rotated with respect to the polarization vector of theincident pulse. This polarization rotation causes a portion of pulse 807of FIG. 8B to be directed out of the laser cavity of FIG. 7A by thepolarized beam splitter 719 along the direction 731 of FIG. 7A.

In this manner, at least a portion of a first ensemble of photons thatconstitutes at least one mode-locked pulse is resolved into two secondensembles of photons with polarization vectors different from saidensemble of photons and since they have polarization vectors that aredifferent from the polarization vector of the lasing radiation, at leasta portion of at least one of the second ensembles of photons will bedirected by the polarized beam splitter 719 out of the laser cavity.This is also referred to as dumping a pulse from the laser cavity.

The single pulse dumped from the laser cavity of FIG. 7A is depicted as811 of FIG. 8D. The extended high modulation voltage 809 can occurperiodically or can occur according to a selected pattern. For example aperiodic repetition of the extended high voltage 809 every N cycles ofthe modulating signal would produce a pulse train with a repetition rateN times lower than the repetition rate of the original mode-locked pulsetrain depicted in FIG. 8A.

Such reduction in pulse train repetition is useful in an embodiment thatconsists of a compact mode-locked laser which generates a highrepetition rate mode-locked pulse train (because of its short round triptime) but enables using conventional detection and electronic techniquesthat typically operate at lower repetition rates and thereby reduces theelectronic bandwidth requirements of the system.

Such reduction in pulse train repetition rate is also useful ingenerating very low repetition rate pulse trains similar to thoseachieved by conventional cavity dumping techniques. However, with thepresent invention, a very compact cavity dumped mode-locked laser can beachieved.

It should be understood that the diagram depicted in FIG. 7A is asimplified diagram of a mode-locked laser for illustrative purposes. Anactual mode-locked laser, such as a Ti:saphire (titanium:sapphire)mode-locked laser, may have additional optical elements, such asfocusing elements and dispersive elements. Such dispersive elements maybe prisms or mirrors.

It should also be understood that the applications of the inventiondescribed herein are to illustrate the invention and not intended tolimit the scope or application of the invention. In addition to opticalswitching or modulation for: optical communications; optical datastorage; mode-locking or cavity dumping lasers; the invention issuitable for other applications including, but not limited to, displaysystems, such as video display systems.

It should further be understood that the above description is intendedto be illustrative and not restrictive. Many variations and combinationsof the above embodiments are possible. Many of the features havefunctional equivalents that are intended to be included in the inventionas being taught and many other variations and combinations of the aboveembodiments are possible, for example, various combinations ofbeam-splitters can be used, including but not limited to: cube beamsplitters; plate beam splitters.

The preferred embodiments illustrated are free space configurations.Equivalent configurations could be implemented in optical fiber or incombinations of free space and optical fiber. In such designs orconfigurations beam-splitters could be replaced by fiber couplers.Equivalent configurations could be implemented using wave-guidetechniques.

The invention applies to all regions of the electro-magnetic spectrum,including but not limited to, micro-wave, infra-red, visible,ultra-violet, X-ray, or gamma ray and is not restricted to the regionconventionally referred to as optical.

Optical sources, include but are not limited to, light emitting diodes(LED); superluminescent diodes (SLD); laser sources; laser diodes; fiberlasers; wavelength tunable laser diodes; swept source lasers;mode-locked lasers; and continuum generating sources.

The preferred embodiments include collimated optical sources, however,they could also various combinations of lenses or lens arrays could beemployed to collimate the radiation or to focus the radiation onto thedynamically configurable reflective element.

In the preferred embodiments, the asymmetric property that resolves theincident ensemble of photons into two ensembles of photons withdifferent polarization vectors is a diffractive element that isdynamically induced by means electric currents, i.e. the diffractiveelement is electronically induced. Furthermore, in the preferredembodiments the induced diffractive element is normal to the incidentradiation.

In other embodiments the induced diffractive element could be alignedwith the incident radiation in a manner other than normal, for example,in a plane parallel to the incident radiation. In other embodiments adynamically induced asymmetric property could be a diffractive elementinduced by means other than electric currents, for example, by magneticmeans, or by an optical interference based fringe pattern, i.e. by anoptical interference signal.

In other embodiments a dynamically induced asymmetric property need notbe based on inducing a diffractive element, but could be any asymmetricproperty that can be dynamically induced and that resolves an incidentensemble of photons into one or two ensembles of photons where thepolarization vectors of the one or two resolved ensembles are differentfrom each other and different from the polarization vector of theincident ensemble.

In the preferred embodiments, the polarization vector of the incidentradiation on the dynamically configurable reflective element issubstantially azimuthally at 45 degrees to the direction of thedirection of the diffractive element. In other embodiments, thepolarization vector of the incident radiation on the dynamicallyconfigurable reflective element can be at angles different from 45degrees to the direction of the direction of the diffractive element.

In the preferred embodiments, the two resolved ensembles of photons arereflected by a reflective structure along the direction of incidence ofthe incident ensemble of photons. In other embodiments the two resolvedensembles of photons could be reflected at one or more angles withrespect to the direction of incidence of the incident ensemble ofphotons or could be transmitted through the structure along thedirection of incidence or transmitted at one or more angles with respectto the direction of incidence of the incident ensemble of photons.

In the preferred embodiments the polarization vector of the incidentensemble of photons can be considered as a quantum state of the incidentradiation. The reflective element with the dynamically configurableasymmetric property leaves the quantum state of the ensemble of photonsunchanged when in its symmetric configuration.

In its asymmetric configuration, the reflective element with thedynamically configurable asymmetric property switches the quantum stateof the incident photons resulting in one or two ensembles of photonswith quantum states different from the quantum state of the incidentphotons.

In other embodiments a quantum state other than one related to apolarization vector could be switched by a dynamically configurableproperty other than a reflective element with a dynamically configurablediffractive element. In general the dynamically configurable propertycan switch a quantum state of ensemble of photons to at least onedifferent quantum state when in an asymmetric configuration and leavethe quantum state substantially unchanged when in its symmetricconfiguration.

An example of an embodiment that is a combination of the above describedexamples is illustrated and described with respect to FIG. 9A. Thisembodiment again includes an optical source 901, that generatescollimated linearly polarized electro-magnetic radiation 903 that isapplied to an optional polarization optic 905. The optional polarizationoptic 905 is a polarizer that enhances the degree of linear polarizationof the radiation 903. The optional polarization optic 905 may alsoinclude a wave-plate to rotate the plane of polarization.

The linearly polarized radiation 907, is applied to a dynamicallyconfigurable element 909 that has a property that can be dynamicallyconfigured by an optical signal. In a first configuration of thedynamically configurable element 909, the dynamically configurableproperty is symmetric. In this symmetric first configuration ittransmits incident radiation 907 with no effect on the linearpolarization vector of the radiation 907.

In this first configuration of the dynamically configurable element 909,the output radiation 911, with no modification of polarization vector,is applied to a polarized beam splitter 913 which is azimuthallyoriented to direct substantially all the radiation 911 transmitted withthe dynamically configurable element 909 in the (symmetric) firstconfiguration in the direction indicated by the arrow 915.

In this embodiment the dynamically configurable property of thedynamically configurable element 909 is an optically induced diffractiveelement. Such a diffractive element may be induced in the dynamicallyconfigurable element 909 by the imposition of an optical interferencebased fringe pattern. Such an interference based fringe pattern may, forexample, be generated by directing two coherent laser beams 917 and 919at the dynamically configurable element 909.

The angles of incidence and wavelength of the two coherent laser beamscan be selected to generate an interference based fringe pattern with aspatial periodicity designed to optimize diffraction of the wavelengthof the radiation 911. There are many conventional techniques forgenerating such a fringe pattern based on well known interferenceeffects.

In the case of the imposed interference based fringe pattern beinginduced in a non-conductive layer (transparent at the wavelength of theincident radiation 907), the fringe pattern causes a perturbation of thecharge distribution thereby inducing a diffractive element correspondingto the imposed optical interference based fringe pattern. In otherembodiments the interference based fringe pattern could induce magneticor electric fields or electric currents that would constitute a periodicperturbation.

A representation of such an imposed interference based fringe pattern,that constitutes one configuration of the dynamically configurableelement 909, is depicted in (the dashed circle) FIG. 9B. The azimuthallyasymmetric pattern 925 is azimuthally aligned at substantially 45degrees with respect to the azimuthal direction 927 of the polarizationvector of the radiation 907.

In this asymmetric configuration of the dynamically configurable element909, the induced diffractive element 925 resolves the ensemble ofphotons that constitute radiation 907 with polarization vector 933 ofFIG. 9C (in dashed box 931) into two ensembles of photons withpolarization vectors 935 and 937 of FIG. 9C that propagate at least inpart as radiation 911.

When the two ensembles of photons with polarization vectors 935 and 937encounter the polarized beam splitter 913. A component of each of thetwo ensembles of photons with polarization vectors 935 and 937 areresolved propagates through the polarized beam splitter 913 as outputradiation 921.

Thus the presence or absence of the interference pattern 925 determinesthe presence or absence of the output radiation 921, i.e. the outputradiation 921 is switched on or off by the presence or absence of theinterference pattern.

More generally FIG. 9A depicts a system for modifying a quantum state ofelectromagnetic radiation comprising an optical source that generateselectromagnetic radiation with a first linear polarization vector whichconstitutes a first ensemble of photons with a first quantum state and atransmissive optical element upon which a dynamically configurableoptically induced diffraction grating can be imposed which constitutes atransmissive element with a dynamically configurable property to whichthe above mentioned radiation, i.e. the first ensemble of photons isapplied.

The optical interference fringe pattern that optically induces thediffractive pattern is generated by convention optical interferencemeans using one or more coherent laser sources and constitutes a signalgenerator that generates the signal that is applied to the transmissiveelement with a dynamically configurable property, i.e. the diffractiveelement that can be imposed.

The presence of the optically induced diffractive grating causes atleast a portion of the incident electromagnetic radiation with a firstlinear polarization vector (i.e. the first ensemble of photons with afirst quantum state) to be resolved into two ensembles of photons withpolarization vectors different from the first linear polarization (i.e.the first ensemble of photons with a first quantum state is switched toat least one second ensemble of photons with a quantum state differentfrom the quantum state of said first ensemble of photons.

The polarized beam splitter 913 enables only components of the resolvedensembles of photons with polarization vectors orthogonal to thepolarization vector of the first ensemble of photons to be output. Inanother embodiment the polarized beam splitter 913 could be replaced bya conventional polarizer.

The relative phases of the two resolved ensemble of photons could becontrolled to have a desired relationship (such as being coincident) bydesigning the optical element 909 (upon which the diffractive element isimposed) to have bulk birefringent properties for vectors 935 and 937such that the desired relationship is achieved.

Rather than a system that includes an optical source 901 (and possiblythe optional element 905) a device could be configured that wascomprised of: (a) the dynamically configurable element; (b) the signalgenerator for generating and applying the interference fringe pattern;and (c) a polarizing output element, either polarized beam splitter,polarizer, or an equivalent.

Other examples will be apparent to persons skilled in the art. The scopeof this invention should be determined with reference to thespecification, the drawings, the appended claims, along with the fullscope of equivalents as applied thereto.

What is claimed is:
 1. A system for modifying a quantum state ofelectromagnetic radiation, said system comprising: an optical sourcethat generates a first ensemble of photons with a first quantum state; areflective element with a dynamically configurable property to whichsaid ensemble of photons is applied; a signal generator with which asignal is generated and applied to said reflective element with adynamically configurable property, such that at least a portion of saidfirst ensemble of photons with a first quantum state is switched to atleast one second ensemble of photons with a quantum state different fromthe quantum state of said first ensemble of photons.
 2. A system formodifying a quantum state of electromagnetic radiation, said systemcomprising: an optical source that generates a first ensemble of photonswith a first quantum state; a transmissive element with a dynamicallyconfigurable property to which said ensemble of photons is applied; asignal generator with which a signal is generated and applied to saidtransmissive element with a dynamically configurable property, such thatat least a portion of said first ensemble of photons with a firstquantum state is switched to at least one second ensemble of photonswith a quantum state different from the quantum state of said firstensemble of photons.
 3. A device for modifying a quantum state ofelectromagnetic radiation, said device comprising: a transmissiveelement with a dynamically configurable property to which said a firstensemble of photons with a first quantum state is applied; a signalgenerator with which a signal is generated and applied to saidtransmissive element with a dynamically configurable property, such thatat least a portion of said first ensemble of photons with a firstquantum state is switched to at least one second ensemble of photonswith a quantum state different from the quantum state of said firstensemble of photons.
 4. The device of claim 3, further including apolarizing element configured to output one or more components of thesaid at least one second ensemble of photons with a polarization vectororthogonal to the polarization vector of said first ensemble of photons.5. A device for modifying a quantum state of electromagnetic radiation,said device comprising: a reflective element with a dynamicallyconfigurable property to which said a first ensemble of photons with afirst quantum state is applied; a signal generator with which a signalis generated and applied to said reflective element with a dynamicallyconfigurable property, such that at least a portion of said firstensemble of photons with a first quantum state is switched to at leastone second ensemble of photons with a quantum state different from thequantum state of said first ensemble of photons.
 6. The device of claim5, further including a polarizing element configured to output one ormore components of the said at least one second ensemble of photons witha polarization vector orthogonal to the polarization vector of saidfirst ensemble of ph